1. Field of the Invention
The present invention relates to an electronic ignition timing control system for electrically determining the ignition timing of an internal combustion engine.
2. Description of the Prior Art
In the past, conventionally used distributors for determining the ignition timing of internal combustion engines have been so designed that the relative positions of the cam and the elements in the electric contact unit employing points are changed in accordance with the engine rotational speed and the intake manifold vacuum respectively detected by the centrifugal governor and the vacuum advancer, and an ignition signal is generated at the correct ignition point that suits the operating conditions of the engine.
However, since the above-mentioned conventional systems are all controlled mechanically, the systems are disadvantageous in that the ignition timing tends to be inaccurate and it is difficult to ensure the ideal ignition timing characteristic, thus making the systems unsuitable from the standpoint of exhaust emission control which has presently become an issue.
Although electronic ignition systems have also been proposed in which the ignition timing is electronically determined by dividing the initially set ignition advance angle by the engine speed and converting it into time, a disadvantage of this type of conversion systems is that since the number of engine revolutions must be computed thus requiring the time for computing it, the value of the engine speed represents the average number of the engine revolutions within the computing time, and consequently any variation in the engine revolutions during the computing time causes an error. Further, though a system has been proposed in which in consideration of the above-mentioned difficulty a rotor disk is formed along its circumference with slits each corresponding to a predetermined crank angle and the slits are detected to determine the ignition timing, this system is not suited for use in practical application in consideration of the manufacturing capacity and the capacity and durability of sensors for detecting the slits, since in practice the rotor disk must be formed with 180 slits in case the slits are provided at intervals of 2.degree. of crank angle and the rotor disk is mounted on the crankshaft.